Ribosomes and tRNAs build protein molecules according to the sequences of mRNA molecules. They don't know what they are building; whatever the mRNA sequence, they build the corresponding amino acid polymer.

The primary determinant of protein folding is the hydrophobic effect, in which hydrophobic amino acids are forced to the center, and hydrophilic amino acids cover the exterior of the protein "globule." Subsequent events fine-tune the structure and adopt the lowest-energy conformation. As with transcription and translation, these events know nothing. They simply occur, with the results dependent on the sequence of amino acids.

Regulation of Gene Expression

Some genes are expressed in all cells of a multicellular organism. Other genes are expressed only in specific locations. Many genes that determine the patterns of embryonic growth and development are expressed only in small numbers of cells. Mutations that change their patterns of expression can have dramatic effects on the morphology of the fully-developed organism.

Genetics

Transmission of Information From
Generation to Generation

Most simply put, every species reproduces after its own kind. Offspring inherit DNA from their parents. Yes, there is meiotic shuffling of alleles, and there are dominant/recessive allelic relationships, but the essential fact is that DNA is inherited. Mutations are inherited (when they occur in gametes). Individual multicellular organisms cannnot change their genetics during their lifetime, so evolutionary change can occur only by descendent generations replacing ancestral generations.

The specific alleles an individual organism inherits from its parents determine its heritable characteristics. It is important to have a sense of how this can occur, rather than merely memorizing the terms "genotype" and "phenotype." Two models that are accessible are pigmentation (such as fly eyes and hair color) and limb development (see below).

Most developmental decisions occur in small groups of cells that are precursors to the larger structures of the mature individual. These decisions involve cell-cell interactions, diffusible molecules (what we might call "short-range hormones"), and changes in the patterns of gene expression.

We know many of the genes that are involved in limb development, and can therefore begin to visualize how modest changes in their expression can create significant changes in the morphology of the mature animal.

Natural Selection favors some genetic variants, and selects against others. Students often see this as a vague statement. The statement loses some of its vagueness when we rephrase it in terms of ecological competition. Those who out-compete the others are the ones whose genes are passed on.

Competition occurs, of course, because resources are limited. At equilibrium, most ecosystems are at their carrying capacity for the species within them. For one species to prosper, or for some genetic variants within a species to prosper, they do so at the expense of others that are less fit.